1. Field of the Present Invention
The present invention relates to an optical information recording medium, and more particularly to an optical information recording medium capable of recording and replaying information by irradiation of laser light.
2. Description of the Related Art
A high-density optical information recording medium (DVD) that records and reproduces a large amount of text information, image information and sound information is increasingly demanded. In particular, in order to accommodate picture recording for digital HDTV broadcast, further increase of a recording density of an optical recording medium is earnestly studied. Increase of the recording density has been attained, for example, by shortening a wavelength of laser light used for recording and replaying on an optical recording medium, or by increasing an aperture (NA) of an objective lens of an optical system of an apparatus for recording and replaying. Under these circumstances, blue-violet lasers have been put on the market.
There is a phase change-type optical disk known as a DVD, which employs an alloy layer, such as GeSbTe, as a recording layer. The recording layer is heated for a moment with laser light to cause a phase change from a crystalline state to an amorphous state. A change in reflection coefficient due to the phase change is utilized for recording and replaying. In recent years, a DVD system has been reported (xe2x80x9cISOM2000xe2x80x9d, pp. 210 to 211) in which recording and replaying are carried out with a blue-violet laser by using the phase change-type recording layer. The demand for high density recording has been attained to a certain extent by this system, but there is a problem that the durability of the recording layer to repeated replaying is insufficient.
A recordable DVD (DVD-R) that can record information only once is also known among DVDs. The DVD-R has a basic constitution of an optical disk including a substrate having grooves and, formed thereon, a recording layer, a reflective layer and a protective layer. In general, structures are known in which such optical disks each having a substrate with a reflective layer and a protective layer are attached to each other at the sides of the protective layers with an adhesive, or such an optical disk and an optical disk formed with a single substrate are attached to each other. Additionally, a constitution is also known in which a reflective layer, a recording layer and a cover layer are provided on a substrate in this order, and recording and replaying with laser light are carried out on the side of the cover layer.
Red laser light having a wavelength of 635 or 650 nm is generally used for recording and replaying of the DVD-R. However, there is an increasing demand for recording density on DVD-Rs. In order to attain high density recording, recording and replaying with laser light having a shorter wavelength instead of the red laser light become necessary. In order to attain high density recording of the DVD-R, the use of laser light of a shorter wavelength and the use of an objective lens of a higher NA have been considered.
In general, the energy of light increases inversely proportionally to the wavelength, and when the wavelength of laser light is decreased for high density recording of an optical disk, the light energy applied to the optical disk is increased. In particular, DVD-R uses a layer of an organic substance as a recording layer, and therefore the recording layer is deteriorated upon repeated irradiation with light for replaying (replaying deterioration), causing a problem in durability. Reduction of the laser output has been considered, but the reflection is lowered when the replaying output is decreased, which causes problems of reduction in RF output, reduction in C/N (carrier to noise ratio), and reduction in reproducing durability.
Another optical information recording medium (optical disk) that can record information only once with laser light has also been known. This optical disk is referred to as a recordable CD (xe2x80x9cCD-Rxe2x80x9d), and a representative structure thereof includes a transparent disk substrate having laminated thereon a recording layer formed with an organic substance, a reflective layer formed with a metal such as gold, and a protective layer formed with resin, in this order. Recording of information on the CD-R is carried out by irradiation with laser light in the near infrared region (which is generally laser light having a wavelength around 780 nm), and the recording is attained by a mechanism in which the temperature of an irradiated part of the recording layer is locally increased by absorbing the light, and the optical characteristics of that part are changed by a physical or chemical change (such as formation of a pit), so as to attain recording of information. The reading of information (replaying) is carried out by irradiation on the CD-R with laser light of the same wavelength as the laser light for recording, and a change in reflectivity between parts of the recording layer having the changed optical characteristics (recorded part) and parts having the unchanged optical characteristics (unrecorded part) is detected, so as to attain replaying.
In recent years, an optical information recording medium having a higher recording density has been demanded. In order to deal with this demand, the optical disk referred to as a recordable digital versatile disk (xe2x80x9cDVD-Rxe2x80x9d) has been demanded (described, for example, in xe2x80x9cNIKKEI NEW MEDIAxe2x80x9d, additional volume xe2x80x9cDVDxe2x80x9d, published 1995). In the structure of the DVD-R, the two disks, each containing a transparent disk substrate, generally have guide grooves (also referred to as grooves or pregrooves) for tracking irradiated laser light which are formed with a small half-value width (0.74 to 0.8 xcexcm), which is less than a half of that of the CD-R, and a recording layer containing an organic substance, the reflective layer and protective layer laminated on the substrate in this order, and are attached to each other with the recording layers turned inside, or are attached to each other with the recording layers turned inside with a disk protective substrate interposed therebetween. Because recording and replaying of the DVD-R is carried out by irradiating with visible laser light (which is generally laser light having a wavelength in a range from 630 to 680 nm), recording with a higher density than the CD-R can be attained.
Networks, such as the Internet, and high resolution TV are quickly spreading these days. Test broadcasts of HDTV (high definition television) have recently started. Under the circumstances, a recording medium of a large capacity is demanded that can easily record image information at low cost. The DVD-R sufficiently achieves the role of the large capacity recording medium under the current situation, but there is constantly increasing demand for larger capacity and higher density recording media, and it is necessary to develop a recording medium that can deal with these demands. Accordingly, development of a recording medium having a larger capacity, which can attain high density recording with light having a shorter wavelength than that for the DVD-R, is being continued.
For example, JP-A No. 4-74690, No. 7-304256, No. 7-304257, No. 8-127174, No. 11-53758, No. 11-334204, No. 11-334205, No. 11-334206, No. 11-334207, No. 2000-43423, No. 2000-108513, No. 2000-113504, No. 2000-149320, No. 2000-158818 and No. 2000-228028 disclose methods for recording and replaying, in which an optical information recording medium having a recording layer containing an organic substance is irradiated with laser light of a wavelength of 530 nm or less from the side of the recording layer toward the reflective layer, so as to attain recording and replaying of information. In these methods, an optical disk having a recording layer containing a substance such as a porphyrin compound, an azo organic substance, a metallic azo organic substance, a quinophthalone organic substance, a trimethylcyanine organic substance, an organic substance having a dicyanobiphenyl skeleton, a coumarin compound or a naphthalocyanine compound is irradiated with blue laser light (wavelength: 430 nm or 488 nm) or blue-green laser light (wavelength: 515 nm), so as to attain recording and replaying of information.
In order to attain compatibility with the CD-R system that is currently used, an optical information recording medium has been proposed that can attain recording and replaying with two kinds of laser light having different wavelengths. For example, JP-A No. 2000-141900, No. 2000-158816, No. 2000-185471, No. 2000-289342 and No. 2000-309165 propose such optical information recording media that can attain recording and replaying with either laser light of the near infrared region around 780 nm or visible laser light around 650 nm by using a mixture of an organic material used in CD-R and an organic material used in DVD-Rs.
However, according to investigations performed by the present inventors, the optical disks disclosed in the publications noted in the foregoing require further improvements, because they cannot provide the sensitivity that is practically demanded to record information by irradiation of laser light of a short wavelength from 380 to 500 nm, and other recording characteristics, such as reflectivity and a modulation degree, are also insufficient. In particular, the optical disks disclosed in the publications noted in the foregoing show lowered recording characteristics upon irradiation with laser light having a wavelength of from 380 to 500 nm.
The present invention has been developed taking the foregoing problems into consideration, and an object thereof is to provide an optical recording medium that is excellent in both C/N (carrier to noise ratio) and reproducing durability and can attain high density recording without any problem.
Another object of the present invention is to provide an optical information recording medium that can attain recording and replaying in a high density by irradiation of blue laser light of a short wavelength of from 380 to 500 nm to exhibit excellent recording characteristics.
The foregoing objects are attained by the following optical recording media of the present invention.
The present invention relates to, as a first aspect, an optical recording medium including a substrate including an organic substance with at least one maximum absorption peak in each of a range from 600 to 800 nm and a range from 300 to 400 nm, the substrate including a groove that includes a track pitch of from 200 to 400 nm, a groove depth of from 50 to 150 nm and a groove half-value width of from 90 to 200 nm; a reflective layer on the substrate; a recording layer on the reflective layer; and a cover layer on the recording layer, wherein the medium is recordable and replayable using laser light including a wavelength of 450 nm or less and an objective lens including an aperture NA of 0.7 or more.
The present invention also relates to, as a second aspect, an optical recording medium including a substrate including an organic substance with at least one maximum absorption peak in each of a range from 600 to 800 nm and a range from 300 to 400 nm, the substrate including a groove that includes a track pitch of from 200 to 400 nm, a groove depth of from 50 to 150 nm and a groove half-value width of from 90 to 200 nm; a reflective layer on the substrate; and a recording layer on the reflective layer, wherein the medium is recordable and replayable using laser light including a wavelength of 450 nm or less and an objective lens including an aperture NA that is at least 0.5 and less than 0.7.
The present invention further relates to, as a third aspect, an optical information recording medium including a substrate including a groove depth of from 15 to 45 nm, a track pitch of from 250 to 400 nm, a groove half-value width of from 60 to 200 nm and a groove tilt angle of from 40xc2x0 to 80xc2x0; a reflective layer on the substrate; a recording layer including an organic substance on the reflective layer, at which recording layer information is recordable and replayable by irradiating laser light including a wavelength of from 380 to 500 nm through a lens including an aperture NA of 0.7 or more; and a sheet layer on the recording layer.